The power supply installed inside a computer is to transform AC power into stable and different voltage level DC powers for the devices inside the computer, such as the mother board, hard drives, and optical drives. In the earlier stage, the power supply cannot provide power for the devices inside the computer unless the computer is turned on.
In the currently prevailing design of the ATX standard, besides the DC powers (3.3V, 5V, 12V, etc.) provided when the computer is turned on, there is also a regular power which can supply power no matter whether the computer is turned on or turned off.
Besides the operating system and application programs, many external electronic devices are also used in the present application environment of computers, including: speakers, night lamps, the CD drives capable of replaying music directly, USB charging devices for PDA or mobile phones, the minor LCD on the computer casing for presenting time, temperature, humidity, etc., and the gorgeous light-emitting computer casing. All the abovementioned external electronic devices can operate independently without the operating system. However, in many present designs, power supplies cannot supply power unless the computer is turned on. Alternatively, a regular power system is installed in a power supply to provide DC power when the computer is turned off. However, the wattage output by a regular power system is smaller, and the number of voltage levels is insufficient. Therefore, the conventional regular power system is unlikely to supply sufficient power for a high-power external electronic device.